Method and apparatus for ion energy distribution manipulation for plasma processing chambers that allows ion energy boosting through amplitude modulation

ABSTRACT

Methods and apparatus for boosting ion energies are contemplated herein. In one embodiment, the methods and apparatus comprises a controller, a process chamber with a symmetrical plasma source configured to process a wafer, one or more very high frequency (VHF) sources, coupled to the process chamber, to generate plasma density and two or more frequency generators that generate low frequencies relative to the one or more VHF sources, coupled to a bottom electrode of the process chamber, the two or more low frequency generators configured to dissipate energy in the plasma sheath, wherein the controller controls the one or more VHF sources to generate a VHF signal and the two or more low frequency sources to generate two or more low frequency signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of and claims the benefit of U.S.patent application Ser. No. 15/449,456, filed Mar. 3, 2017 which isherein incorporated by reference in its entirety.

FIELD

Embodiments disclosed herein generally relate to a method and apparatusfor plasma-assisted substrate processing techniques.

BACKGROUND

In the semiconductor manufacturing industry, the size of features etchedon substrates, such as semiconductor wafers, continues to decrease andtransistor structures are becomes increasingly complex. For example,there is a growing trend of forming a chain of transistors verticallyinstead of laterally, as is the case in vertical NAND memory structures.These vertical structures come with their own unique challenges becausevery high aspect ratio holes must be made in order to make contacts ordeep trenches so that the infrastructure for electrical pathways can belaid.

The etching of these high aspect ratio holes calls for the use of highion energies in an abundant supply (ion flux). It is important that theholes do not bend or twist while etching and that the holes maintainconsistency even when the holes become deeper without significant lossof etch rate. Solutions that are typically employed have severalproblems. First, existing solutions often apply frequencies to the samedriven electrode closely couples ion energies and ion flux byinfluencing the plasma sheath. As a practical problem coupling allfrequencies to the same driven electrode requires higher and higher lowfrequency power (responsible for ion energies) when higher flux (higherVHF) is required. The second problem with existing technologies is thatthe thermal burden on the transmission lines to the chamber and wafercarrying substrate is excessive.

Therefore, the inventors have provided methods and apparatus for plasmaprocessing that decouple the ion energies and the ion flux and thatreduce the thermal burden on transmission lines in plasma processing.

SUMMARY

Methods and apparatus for plasma processing using low frequency ionenergy boosting are provided herein. In some embodiments, an apparatusfor boosting ion energy includes a controller, a process chamber with asymmetrical plasma source configured to process a substrate (e.g., awafer), one or more very high frequency (VHF) sources, coupled to theprocess chamber, to generate plasma density and two or more frequencygenerators that generate low frequencies relative to the one or more VHFsources, coupled to a bottom electrode of the process chamber, the twoor more low frequency generators configured to dissipate energy in theplasma sheath, wherein the controller controls the one or more VHFsources to generate a VHF signal and the two or more low frequencysources to generate two or more low frequency signals.

In other embodiments, a method for plasma processing using low frequencyion energy boosting comprises processing a wafer using a symmetricalplasma source in a process chamber, providing two or more frequenciesless than a predetermined threshold for dissipating energy in a plasmasheath, the frequencies provided to a bottom electrode of the processchamber, providing one or more flux generating frequencies to thesymmetrical plasma source, and providing a low impedance DC ground forbias frequencies.

Other and further embodiments are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure, briefly summarized above anddiscussed in greater detail below, can be understood by reference to theillustrative embodiments of the disclosure depicted in the appendeddrawings. However, the appended drawings illustrate only typicalembodiments of the disclosure and are therefore not to be consideredlimiting of scope, for the disclosure may admit to other equallyeffective embodiments.

FIG. 1 is a block diagram of a process chamber for boosting ion energiesin accordance with exemplary embodiments disclosed herein;

FIG. 2 is a more detailed block diagram of the upper portion of theprocess chamber of FIG. 1 in accordance with exemplary embodimentsdisclosed herein;

FIG. 3 is a block diagram of a controller in accordance with exemplaryembodiments of the present disclosure;

FIG. 4 is a flow diagram of a method for boosting ion energies inaccordance with exemplary embodiments disclosed herein;

FIG. 5 depicts several waveforms that can be provided to processchambers disclosed herein;

FIG. 6 depicts ion energy distributions according to exemplaryembodiments disclosed herein;

FIG. 7 illustrates multiple graphs of ion energy distributions inaccordance with exemplary embodiments disclosed herein;

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. The figures are not drawn to scale and may be simplifiedfor clarity. Elements and features of one embodiment may be beneficiallyincorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

Embodiments of a method and apparatus for ion energy boosting areprovided herein. According to one embodiment, an apparatus forprocessing substrates, such as semiconductor wafers, is provided. Theapparatus comprises a symmetrical VHF plasma source coupled to a topelectrode and a bottom electrode coupled to two or more low frequencysources that dissipate power in the plasma sheath rather than the bulkplasma. The two or more low frequencies are selected such that theyprovide sufficient amplitude modulation, e.g., a modulation depth of 0.2or more but less than 1 to provide higher peak to peak voltages whenthese two low frequencies are used in combination, rather than lowfrequencies individually. The symmetrical VHF plasma source provides lowimpedance return through a top electrode for the low frequencies appliedto the bottom electrode, while the top electrode is RF hot in order forthe one or more VHF frequencies to generate plasma density. The lowimpedance ground of the top electrode allows the bias currents to returnback to the low frequency generators In some embodiments, a processchamber further comprises an auxiliary electrode that is sensitive tothe VHF applied to the top electrode, or that is sensitive to resonantfrequencies of the two or more low frequencies applied to the bottomelectrode.

FIG. 1 is a block diagram of an apparatus 100 for boosting ion energiesin accordance with exemplary embodiments.

The apparatus 100 comprises a VHF source 101, a VHF match 102, a processchamber 111, a first frequency generator 140, a second frequencygenerator 150, a first low frequency match 104, a second low frequencymatch 122, a first isolator 110, a second isolator 112 and a controller170. In some embodiments, a high voltage (HV) probe 130 is coupled tothe bottom to the bottom electrode 106.

The process chamber 111 comprises one or more symmetrical conductors103, a top electrode 105, a bottom electrode 106, an electrostatic chuck109 for supporting a wafer 107.

The VHF source 101 generates a very high frequency (VHF) signalapproximately greater than 30 MHz and approximately less than 300 MHz.The VHF source 101 is coupled to a VHF match 102 via a transmissionline. The VHF match 102 is an impedance matching circuit which matchesthe impedance of the VHF source 101 with the impedance of the load,e.g., the process chamber 111 in order to maintain maximum powertransfer over the transmission line. The VHF source 101 is responsiblefor increasing plasma density and, therefore inducing ion flux in theprocess chamber 111. In some embodiments, there is more than one VHFsource coupled to the process chamber 111. In such embodiments, each VHFsource is coupled to a respective matching circuit, the output of whichis coupled to the process chamber 111.

The process chamber 111 contains a symmetrical plasma source, i.e., aplurality of symmetrical conductors collectively referred to assymmetrical conductors 103 which deliver RF power to the top electrode105 in a uniform symmetrical manner. The symmetrical conductors 103 arecontained within a hollow cylinder 159. The hollow cylinder 159 isconnected to the RF power delivery apparatus 180, consisting of aplurality of cylindrical metal structures (at least three). In theexample of three hollow cylinders, the center cylinder 160 and the innercylinder 162 are connected to each other and the top electrode 105 atthe top electrode 105. The inner cylinder 162 is connected on the top tothe outer cylinder 164 by an annular ring 182. The symmetricalconductors 103 are connected to the center cylinder 160 through anaperture hole 184 in the annular ring 182. Since all the cylinders areall connected to the chamber ground, the top electrode 105 is connectedto the chamber ground which is at DC potential providing a low impedancebias current return.

The symmetrical conductors therefore act as a low impedance groundreturn for low frequency signals generated by the first frequencygenerator 140 and the second frequency generator 150. The top electrode105 is DC grounded to the process chamber 111 via the cylinders, andthus also acts as a good low impedance return for low frequencies.

The wafer 107 to be processed is supported by an electrostatic chuck109. The bottom electrode 106 is below the electrostatic chuck 109 andis coupled to the first frequency generator 140 via the first isolator110 and the first low frequency match 104. The bottom electrode 106 isalso coupled to the second frequency generator 150 via the secondisolator 112 and the second low frequency match 122.

The first low frequency match 104 and the second low frequency match 122are impedance matching circuits that match the impedance of the firstfrequency generator 140 and the second frequency generator 150,respectively, with the load impedance. The first low frequency match 104and the second low frequency match 122 are selected in order tocorrespond with the frequency of the generator they are coupled to. Inan embodiment, for example, the first low frequency match 104 will be a2 MHz match when the first frequency generator 140 is a 2 MHz frequencysignal generator. Similarly, the second low frequency match 122 will bea 400 kHz match when the second frequency generator 150 is a 400 kHzfrequency signal generator.

The first isolator 110 and the second isolator 112 are selected toremove all other frequency signals (reflected signals) except the signalbeing supplied to the isolators directly. For example, if the firstfrequency generator 140 generates a 2 MHz signal, and the secondfrequency generator 150 generates a 400 kHz signal, the first isolator110 will be a 400 kHz RF isolator coupled to a 2 MHz frequency generatorvia a 2 MHz match, while the second isolator 112 will be a 2 MHzisolator coupled to a 400 kHz frequency generator via a 400 kHz match.The isolated signals from the first isolator 110 and the second isolator112 are then coupled to the bottom electrode 106.

According to exemplary embodiments, the probe 130 is a high voltageprobe for measuring voltage at the bottom electrode 106. In embodimentswhere more than two low frequency signals are supplied to the bottomelectrode 106, an equivalent number of isolators (selected forcorresponding frequencies) and an equivalent number of low frequencyimpedance matches (selected for corresponding frequencies) are used. Forexample, if the first frequency generator 140 is a 2

The controller 170 controls the very high frequency signals generated bythe VHF source 101 and the low frequencies generated by the firstfrequency generator 140 and the second frequency generator 150.

According to embodiments, the frequencies generated by the firstfrequency generator 140 and the second frequency generator 150 are atleast an order of magnitude smaller than the frequency of the signalgenerated by VHF source 101. In some embodiments, there are more thantwo frequency generators, thus allowing two or more low frequencyvoltages to be coupled to the bottom electrode 106.

In some embodiments, the first frequency generator 140 and the secondfrequency generator 150 generate a signal (waveform) with a frequencythat is less than 4 MHz. According to one configuration in theembodiment, the first frequency generator 140 generates a signal with afrequency of 2 MHz, while the second frequency generator 150 generates asignal with a frequency of 400 kHz. In other embodiments, thefrequencies may be the same, or may be harmonics of each other. In thecase of harmonics (e.g. the 2 MHz frequency is a 5th harmonic of the 400kHz frequency), phase coherence between both low frequencies becomes anadditional adjustment point used to manipulate ion energies.

The choice of two or more low frequencies less than 4 MHz stronglyamplitudes the lower frequency waveform. For example, the 2 MHz signalwill strongly amplitude modulate the lower frequency of 400 Khz,resulting significantly higher peak voltages on the plasma sheath, andtherefore higher ion energies, when compared to low frequency waveformsoperating at the same power with low peak voltages. The choice of thefrequencies generated by the first frequency generator 140 and thesecond frequency generator 150 are such that they provide sufficientamplitude modulation to provide higher peak to peak voltages when usedin combination, in contrast to using low frequency signals individually.

FIG. 6 illustrates waveforms at various frequencies as measured by probe130. Plot 600 illustrates a waveform at a 2 MHz frequency generating 8KW of power. Plot 602 illustrates a waveform at a 400 kHz frequencygenerating 8 KW of power. Finally, plot 604 illustrates a measurement byprobe 130, where 2 MHz frequency signal amplitude modulates the 400 kHzsignal, each signal having a reduced power of 4 kW, and a peak voltageof 7.310 kV, greater than the individual peak voltages shown in plot 600and 602, or higher voltage for the same power.

FIG. 6 depicts ion energy distributions according to exemplaryembodiments. The “energy boost” attained by the use of two or more lowfrequency signals coupled to the bottom electrode while a VHF signal iscoupled to the top electrode is illustrated along with single frequencynon-modulated signals.

The signals at these frequencies are responsible for energy dissipationin the plasma sheath that is formed on top of the wafer carrying bottomelectrode 106. Frequencies below 4 MHz are deliberately chosen in theembodiment as they are inefficient in plasma generation, and operate onthe plasma sheath, a high impedance load on top of the bottom electrode106. Thus, the frequencies below 4 MHz dissipate power in the plasmasheath and generate high voltage sheaths.

According to the embodiment, the VHF source 101 is decoupled from thelow frequency signals generated by the first frequency generator 140 andthe second frequency generator 150 since the VHF and low frequencysignals are applied to different electrodes and the frequency separationis large. For example, the frequency separation is at least greater thanor equal to 30 MHz. The power requirement to make high voltages issignificantly reduced from configurations where both the low and the VHFsignals are coupled to the same electrode. Consequently, the thermalburden on the transmission lines delivering the power is alsosignificantly reduced in comparison to the single electrode example.

FIG. 2 is a more detailed block diagram of the upper portion of theprocess chamber 111 of FIG. 1 in accordance with exemplary embodiments.

The dashed arrows 200 illustrate the path of the VHF current. The solidarrows 202 indicate the current path of the signals generated by thefirst frequency generator 140 and the second frequency generator 150.Because the top electrode 105 is grounded, the current generated by thefirst frequency generator 140 can use the top electrode 105 using thesolid arrow marks path and return back to the first frequency generator140. Had the top electrode 105 not been grounded, the top electrode 105would not have been in the primary path for the currents generated fromthe first frequency generator 140 to return to the first frequencygenerator 140 and uniformity would have been compromised In someembodiments, the process chamber 111 comprises gas conduits in the upperportion through the DC grounded top electrode.

FIG. 3 is a block diagram of a controller 300 in accordance withexemplary embodiments of the present disclosure.

Various embodiments of methods for boosting ion energies may be executedby the controller 300. Controller 300 is an exemplary embodiment ofcontroller 170 of FIG. 1. According to one embodiment, the controller300 comprises one or more CPUS 1 to N, support circuits 304, I/Ocircuits 306 and system memory 308. The system memory 308 may furthercomprise control parameters 320 and a frequency controller 322. The CPUs1 to N are operative to execute one or more applications which reside insystem memory 308. The controller 300 may be used to implement any othersystem, device, element, functionality or method of the embodimentsdescribed in this specification. In the illustrated embodiments, thecontroller 300 may be configured to implement method 400 (FIG. 4) asprocessor-executable executable program instructions. The frequencycontroller 322 controls the frequency of the signal generators in theapparatus 100 using control parameters 320, where the control parameters320 contain at least one of one or more VHF frequencies, a first lowfrequency (relative to the VHF frequency), and a second low frequency(relative to the VHF frequency). In some embodiments, there are morethan two low frequencies, and these frequencies are stored as controlparameters 320.

In different embodiments, controller 300 may be any of various types ofdevices, including, but not limited to, a personal computer system,desktop computer, laptop, notebook, or netbook computer, mainframecomputer system, handheld computer, workstation, network computer, amobile device such as a smart phone or PDA, a consumer device, or ingeneral any type of computing or electronic device.

In various embodiments, controller 300 may be a uniprocessor systemincluding one processor, or a multiprocessor system including severalprocessors (e.g., two, four, eight, or another suitable number). CPUs 1to N may be any suitable processor capable of executing instructions.For example, in various embodiments CPUs 1 to N may be general-purposeor embedded processors implementing any of a variety of instruction setarchitectures (ISAs). In multiprocessor systems, each of CPUs 1 to N maycommonly, but not necessarily, implement the same ISA.

System memory 308 may be configured to store program instructions and/ordata accessible by CPUs 1 to N. In various embodiments, system memory308 may be implemented using any suitable memory technology, such asstatic random access memory (SRAM), synchronous dynamic RAM (SDRAM),nonvolatile/Flash-type memory, or any other type of memory. In theillustrated embodiment, program instructions and data implementing anyof the elements of the embodiments described above may be stored withinsystem memory 308. In other embodiments, program instructions and/ordata may be received, sent or stored upon different types ofcomputer-accessible media or on similar media separate from systemmemory 308 or controller 300.

In one embodiment, I/O circuits 306 may be configured to coordinate I/Otraffic between CPUs 1 to N, system memory 308, and any peripheraldevices in the device, including a network interface or other peripheralinterfaces, such as input/output devices. In some embodiments, I/Ocircuits 306 may perform any necessary protocol, timing or other datatransformations to convert data signals from one component (e.g., systemmemory 308) into a format suitable for use by another component (e.g.,CPUs 1 to N). In some embodiments, I/O circuits 306 may include supportfor devices attached through various types of peripheral buses, such asa variant of the Peripheral Component Interconnect (PCI) bus standard orthe Universal Serial Bus (USB) standard, for example. In someembodiments, the function of I/O circuits 306 may be split into two ormore separate components, such as a north bridge and a south bridge, forexample. Also, in some embodiments some or all of the functionality ofI/O circuits 306, such as an interface to system memory 308, may beincorporated directly into CPUs 1 to N.

A network interface may be configured to allow data to be exchangedbetween controller 300 and other devices attached to a network, such asone or more display devices (not shown), or one or more external systemsor between nodes. In various embodiments, a network may include one ormore networks including but not limited to Local Area Networks (LANs)(e.g., an Ethernet or corporate network), Wide Area Networks (WANs)(e.g., the Internet), wireless data networks, some other electronic datanetwork, or some combination thereof. In various embodiments, thenetwork interface may support communication via wired or wirelessgeneral data networks, such as any suitable type of Ethernet network,for example; via telecommunications/telephony networks such as analogvoice networks or digital fiber communications networks; via storagearea networks such as Fiber Channel SANs, or via any other suitable typeof network and/or protocol.

Input/output devices may, in some embodiments, include one or moredisplay terminals, keyboards, keypads, touchpads, scanning devices,voice or optical recognition devices, or any other devices suitable forentering or accessing data by one or more controller 300. Multipleinput/output devices may be present or may be distributed on variousnodes of controller 300. In some embodiments, similar input/outputdevices may be separate from biasing controller 300 and may interactwith one or more nodes of controller 300 through a wired or wirelessconnection, such as over a network interface.

In some embodiments, the illustrated computer system may implement anyof the methods described above, such as the methods illustrated by theflowcharts of FIG. 5. In other embodiments, different elements and datamay be included.

FIG. 4 is a flow diagram of a method 400 for boosting ion energies inplasma processing in accordance with exemplary embodiments.

The controller 300 is an exemplary implementation of the method 400 inaccordance with exemplary embodiments of the present disclosure.

The method begins at 402 and proceeds to 404. At 404, the controller 300controls a process chamber (e.g. process chamber 111) to process awafer.

The method proceeds to 406, where the controller 300 controls two ormore frequency generators to generate low frequency signals to dissipateenergy in a plasma sheath above an electrode of the process chamber.

After 406, the proceeds to 408, where the controller 300 controls one ormore VHF sources to generate a flux generating (or VHF) signal with afrequency above a predetermined threshold and coupling that signal tothe symmetrical conductors in the plasma chamber.

The method 400 then proceeds to 410 where the controller 300 provides alow impedance DC ground for the bias frequency signals (e.g., the two ormore low frequency signals). At 410, the method ends. Those of ordinaryskill in the art will recognize that the above-described steps may ormay be performed synchronously, asynchronously, or a combination ofboth, in order or simultaneously.

FIG. 7 illustrates multiple graphs of ion energy distributions inaccordance with exemplary embodiments. The horizontal axis of each graphdepicted represents ion energy up to 10 kEv, while the vertical axisrepresents ion amplitude in arbitrary units. As the ratio of 2 MHz and400 kHz power is varied, the amplitude and ion energy are affectedaccordingly as shown in the graphs.

While the foregoing is directed to embodiments of the presentdisclosure, other and further embodiments of the disclosure may bedevised without departing from the basic scope thereof.

1. A method for plasma processing comprising: processing a wafer using aprocess chamber with a single monolithic top electrode, the singlemonolithic top electrode positioned centrally within the processchamber; providing one or more very high frequency (VHF) signals withone or more VHF sources to the single monolithic top electrode of theprocess chamber via symmetrical conductors; providing two or morefrequency signals having frequencies less than the one or more VHFsignals with two or more frequency generators to a bottom electrode ofthe process chamber, wherein the single monolithic top electrode is DCgrounded to the process chamber such that a symmetrical low impedancecurrent return path for the two or more frequency signals is created;and amplitude modulating a first frequency signal of the two or morefrequency signals with a second frequency signal of the two or morefrequency signals to produce a higher peak-to-peak voltage on a plasmasheath.
 2. The method of claim 1, further comprising: amplitudemodulating the first frequency signal with the second frequency signalto obtain a modulation depth of approximately 0.2 to approximately
 1. 3.The method of claim 1, wherein the second frequency signal has a higherfrequency than the first frequency signal.
 4. The method of claim 1,wherein the second frequency signal is a harmonic frequency of the firstfrequency signal.
 5. The method of claim 1, further comprising:adjusting a phase coherence between the first frequency signal and thesecond frequency signal to manipulate ion energies.
 6. The method ofclaim 1, further comprising: impedance matching the one or more VHFsources and the process chamber using a VHF match; and impedancematching the bottom electrode and the two or more frequency generatorsusing, respectively, two or more frequency matches.
 7. The method ofclaim 6, further comprising: isolating the two or more frequency signalsrespectively using two or more isolators, each isolator of the two ormore isolators is positioned directly between one of the two or morefrequency matches and the bottom electrode and removes frequencies notsupplied directly to the isolator by one of the two or more frequencygenerators.
 8. The method of claim 6, wherein the process chamberfurther comprises: wherein the symmetrical conductors are coupled to theVHF match; and an electrostatic chuck configured to support the wafer,wherein the process chamber is coupled to DC ground.
 9. The method ofclaim 8, wherein the process chamber further comprises: wherein thesymmetrical conductors are connected to a top edge of a hollow cylinderwhich has a bottom edge connected to the single monolithic topelectrode; wherein the hollow cylinder is conductive and RF hot; and anouter hollow cylinder acting as a DC ground, coupled to the singlemonolithic top electrode.
 10. The method of claim 1, wherein the two ormore frequency signals have frequencies that are less than 4 MHz. 11.The method of claim 1, further comprising: generating a signal with aVHF of approximately greater than 30 MHz; and generating a first signalwith a first frequency of 2 MHz and a second signal with a secondfrequency of 400 kHz as frequencies of the two or more frequencysignals.
 12. An apparatus for plasma processing, comprising: acontroller; a process chamber with a symmetrical plasma sourceconfigured to process a wafer with a single monolithic top electrode,the single monolithic top electrode positioned centrally within theprocess chamber opposite a bottom electrode; one or more very highfrequency (VHF) sources, coupled to the single monolithic top electrodevia symmetrical conductors connected to a top edge of a hollow cylinderwhich has a bottom edge connected to the single monolithic topelectrode, to generate plasma density; and two or more frequencygenerators that generate low frequencies relative to the one or more VHFsources, coupled to a bottom electrode of the process chamber, the twoor more frequency generators configured to dissipate energy in a plasmasheath above the bottom electrode, wherein the controller is configuredto control the one or more VHF sources to generate a VHF signal and thetwo or more frequency generators to generate two or more frequencysignals, wherein the controller is configured to amplitude modulate afirst frequency signal of the two or more frequency signals with asecond frequency signal of the two or more frequency signals to producea higher peak-to-peak voltage on a plasma sheath, and wherein the singlemonolithic top electrode is DC grounded to the process chamber such thata symmetrical low impedance current return path for the two or morefrequency signals is provided by the symmetrical conductors.
 13. Theapparatus of claim 12, wherein the controller is configured to amplitudemodulate the first frequency signal with the second frequency signal toobtain a modulation depth of approximately 0.2 to approximately
 1. 14.The apparatus of claim 12, wherein the second frequency signal has ahigher frequency than the first frequency signal.
 15. The apparatus ofclaim 12, wherein the second frequency signal is a harmonic frequency ofthe first frequency signal.
 16. The apparatus of claim 12, wherein thecontroller adjusts a phase coherence between the first frequency signaland the second frequency signal to manipulate ion energies.
 17. Anapparatus for plasma processing, comprising: a controller; a processchamber with a symmetrical plasma source configured to process a waferwith a single monolithic top electrode, the single monolithic topelectrode positioned centrally within the process chamber above a bottomelectrode; one or more very high frequency (VHF) sources, coupled to thesingle monolithic top electrode via symmetrical conductors connected toa top edge of a hollow cylinder which has a bottom edge connected to thesingle monolithic top electrode, to generate plasma density; two or moreisolators for isolating particular frequencies coupled to a bottomelectrode of the process chamber; two or more frequency matches coupledrespectively, to the two or more isolators; and two or more frequencygenerators that each generate a signal with frequencies less than 4 MHz,coupled respectively to the two or more frequency matches, the two ormore frequency generators configured to dissipate energy in a plasmasheath above the bottom electrode, wherein the controller is configuredto control the one or more VHF sources to generate a VHF signal with afrequency greater than 30 MHz and control the two or more frequencygenerators to generate two or more frequency signals, wherein thecontroller is configured to amplitude modulates a first frequency signalof the two or more frequency signals with a second frequency signal ofthe two or more frequency signals to produce a higher peak-to-peakvoltage on a plasma sheath, and wherein the single monolithic topelectrode is DC grounded to the process chamber such that a symmetricallow impedance current return path for the two or more frequency signalsis provided by the symmetrical conductors.
 18. The apparatus of claim17, wherein the controller is configured to amplitude modulate the firstfrequency signal with the second frequency signal to obtain a modulationdepth of approximately 0.2 to approximately
 1. 19. The apparatus ofclaim 17, wherein the second frequency signal has a higher frequencythan the first frequency signal or wherein the second frequency signalis a harmonic frequency of the first frequency signal.
 20. The apparatusof claim 17, wherein the controller is configured to adjust a phasecoherence between the first frequency signal and the second frequencysignal to manipulate ion energies.